1. Field of the Invention
The invention relates to dynamic light scattering, and more particularly to a localized dynamic light scattering measurement system.
2. Description of the Related Art
Laser Doppler velocimetry (LDV) is a well known technique for contactless measurement of the speed and direction of an object in motion, as well as the flow velocity of liquid materials. The motion of a solid object is easy to detect and measure. However, accurate measurements of fluid flow velocity are relatively difficult. Similar to an LDV system, which is used to detect and measure velocity of motions, a dynamic light scattering (DLS) system is also used to characterize motion of particles. But unlike in an LDV system, in which the signal measured is deterministic, the light scattering signal in DLS is a stochastic process that can only be described statistically. Based on detection and measurement of single Rayleigh scattered light from an ensemble of small particles, DLS is a powerful technique to observe the dynamics of particles or molecules undergoing Brownian motion. The dynamic information of these particles can be derived using either autocorrelation or the power spectrum of the intensity time trace signal. The characteristic time of the autocorrelation function or the width of the power spectrum is used to determine the diffusion coefficient of particles in a solution that is proportional to the size of the particles. However, in order to properly extract the dynamic information, assumption of light undergone a limited number of scattering events needs to be satisfied, meaning that each detected photon has been scattered by the sample under a limited number of scattering. Multiple scattering is often encountered in real life situation however, for example, light scattering in tissues. It would be difficult to accurately interpret the dynamic information of a sample if the degree of multiple scattering is severe enough.
FIG. 1 illustrates a conventional dynamic light scattering system proposed in an article by S. Sudol, Y. Miyasaka, K. Otsuka, Y. Takahashi, T. Oishi, and J.-Y. Ko, entitled “Quick and easy measurement of particle size of Brownian particles and planktons in water using a self-mixing laser,” Optics Express, vol. 14, no. 3, pp. 1044-1054, February 2006 The conventional dynamic light scattering system is used to measure the size of particles 84 in Brownian motion, and includes a laser source 81, a beam splitter 82, two acousto-optic modulators (AOMs) 83, a photodetector 85, and a spectrum analyzer 86. The laser source 81 produces a laser beam having an angular frequency at ω0. Almost all the laser beam is transmitted through the beam splitter 82, is then frequency-shifted by the AOMs 83, and is focused onto an object 84 to be measurement containing small particles in suspension by a focusing lens (not shown) such that changing the modulation frequencies of the two AOMs 83 produces a shift of Δω in the carrier frequency at the end of the round-trip. The photodetector 85 receives the rest of the laser beam reflected by the beam splitter 82 and scattered light fed from the moving particles to the laser source 81 to produce an interference signal in the form of an electrical signal that is fed to the spectrum analyzer 86. As such, a power spectrum corresponding to the electrical signal and having a central frequency at Δω is acquired to have a Lorentz profile. The width of the power spectrum is used to determine the diffusion coefficient of the particles that is proportional to the size of the particles. In this case, the self-mixing laser is used.
The self-mixing laser can also be applied to an LDV system. Since Doppler shift is directly proportional to a component of the motion velocity of the object 84 in the direction of incident light, i.e., object motion along the direction of the wave vector can only be measured using a single self-mixing laser, two self-mixing setups are required for measuring in-plane motions of the objects 84. If only a single laser source is utilized for self-mixing, there are multiple beam splitters and multiple pairs of the AOMs 83 required, thereby increasing the overall complexity and loss of light intensity.
If motion/speed of the object 84 is very slow, Doppler shift becomes very small. In this case, interpretation of the center frequency of the power spectrum is difficult even though the power of the incident beam or the high detection sensitivity of the photodetector 85 is used.
Therefore, improvements may be made to the above techniques.